Dissecting high-performance electrohydraulic valves

Things aren't black and white when choosing a servo or proportional valve. Servo and servoproportional valves control pressure or flow and are selected based on application. Proportional and servovalves control flow or pressure via a sliding spool element. First-stage configurations for nozzle flapper and jet-pipe valves. The linear force motor shows permanent magnets, centering springs, coil, and armature. High-performance valves are usually classified as either servo or proportional, a distinction that gives an indication of expected performance. Unfortunately, this classification tends to generalize and blur the true differences between various valve styles. Selection depends on the application and each valve has merit when it comes to controlling pressure (load control) or flow (piston or motor velocity control). Traditionally, the term servovalve describes valves that use closed-loop control. They monitor and feed back the main-stage spool position to the pilot stage or driver either mechanically or electronically. Proportional valves, on the other hand, displace the main-stage spool proportionally to command but they usually do not have any means of automatic error correction (feedback) within the valve. Typically, proportional valves dis-place the spool by driving it against a set of balanced springs, which makes the position proportional to the driving current. These springs also center the main-stage spool. Accuracy and repeatability of the main-stage spool position is a function of the springs' symmetry and ability of the design to minimize nonlinear effects of spring hysteresis, friction, and machining-tolerance variations. To choose the proper hydraulic valve for a specific application, designers must consider specific application and system configurations. Supply pressure, fluid type, system force requirements, valve dynamic response, and